Not Applicable.
The present invention relates generally to optical communication, and more particularly, to optical signal processing utilizing electrical nonlinear transmission line circuits.
Nonlinear transmission lines (NLTLs) for generating fast time domain signals are well known in the art. In general, a voltage-dependent nonlinearity is distributed along a transmission line. One conventional NLTL implementation is an integrated circuit on semiconductor material including diodes and transmission lines. The nonlinear dependence of voltage and reverse-biased capacitance of the diode reduce the transient of a time domain signal applied to the input of the NLTL.
An exemplary NLTL is shown and described in U.S. Pat. No. 5,804,921 to. McEwan et al. Typical NLTL circuits are formed from Gallium Arsenide (GaAs) and can produce transients on the order of several hundred femtoseconds.
GaAs semiconductor material has certain disadvantages compared to the widely known and used Silicon (Si) material, among which are cost and mechanical stability. Si wafers are relatively inexpensive and can be produced at greater sizes, reducing cost of the final integrated circuit. GaAs has the advantage of better drift mobility compared to Si, resulting in shorter transients at the output. However, sub-picosecond transients are not needed for certain applications.
Conventional return-to-zero (RZ) optical transmitters for modulating data streams include two Lithium-Niobate optical modulators, one for generating an optical pulse train and one for modulating the data onto the pulse stream. However, such an arrangement is relatively complex and expensive.
It would, therefore, be desirable to generate a pulse train in the electrical domain using an all-silicon nonlinear transmission line and pulse-forming circuit, modulate the pulse train in the electrical domain with data, and modulate the electrical RZ data stream onto an optical carrier using a single optical modulator.
The present invention provides an integrated nonlinear transmission line and pulse-forming network. The pulse-forming network can include a reverse-biased diode coupled in series with a conductive path of the nonlinear transmission line. With this arrangement, the nonlinear transmission line and the pulse-forming network can be easily integrated in an all-silicon structure.
In one aspect of the invention, an all-silicon integrated circuit includes a nonlinear transmission line and a pulse-forming circuit. In one embodiment, the nonlinear transmission line is monolithically integrated and includes a high-impedance co-planar waveguide and reverse-biased Schottky diodes. The diodes are periodically integrated into the waveguide at predetermined spacings. The pulse-forming network can include a capacitance in the form of a reverse-biased diode coupled to the nonlinear transmission line.
The nonlinear transmission line and the pulse-forming circuit can be integrated in an all-silicon configuration. The circuit can be fabricated using a high yield Si/SiGe, HBT process to provide compatibility with bipolar transistor process technology. With this process, the nonlinear transmission line can be integrated with heterostructure bipolar transistors for high-frequency circuit operation.
In a further aspect of the invention, an electro-optical system provides high-speed signal transmission with optical signal processing. In an exemplary embodiment, a data stream from a nonlinear transmission line and pulse-forming network is provided to an optical modulator that receives a laser signal. The modulator outputs an optical signal, such as return-to-zero pulses, having relatively small, e.g., 27 ps, pulse widths.
The electro-optical system can further include an electrical gate, such as a dual gate transistor, for modulating data, such as data coming from a bit error rate test set, onto an electrical RZ pulse stream. The gate output as well as a laser signal are provided to the optical modulator, which can generate a high-speed, e.g., ten Gbit per second, modulated return-to-zero signal.
In a further aspect of the invention, an electro-optical transmitter is integrated on a silicon substrate. In one embodiment, a nonlinear transmission line, a pulse-forming network, a gate, a laser, and a modulator are disposed on a high-resistivity silicon: substrate. This arrangement facilitates manufacture of the system with concomitant reductions in cost and complexity.